1. Field of the Invention
The present invention relates to a magnetic bearing system employed in a high power (on the order of 100-200 kilowatts) flywheel intended for a hybrid electric vehicle and other applications. More specifically, the present invention relates to a magnetic bearing system employed in a high power flywheel to overcome the stringent demands regarding high electrical power and vehicle motions placed on the stiffness and speed of response of the magnetic bearings while the vehicle is being driven. Moreover, the present invention relates to a magnetic bearing system employed in a high power flywheel producing the long rundown time required, thus providing the extremely low power consumption required, for a parked vehicle. In addition, the present invention relates to a capacitive sensor for the magnetic bearing, which capacitive sensor measures the displacement of a rotor axis from its desired position within a non-rotating assembly of the magnetic bearing.
2. Brief Discussion of Related Art
Modem high strength-to-weight ratio fibers make it possible to construct high energy density flywheels, which, when combined with high power (on the order of 100-200 kilowatts) motor-generators, are an attractive alternative to electrochemical batteries for use as energy buffers in hybrid electric vehicles. A properly designed flywheel system would provide higher power density, higher efficiency, and longer life than a conventional electrochemical battery.
Flywheel energy storage systems have been proposed for many years; many of the storage systems have even been proposed for use in motor vehicles. U.S. Pat. No. 3,741,034, for example, discloses a flywheel contained in an evacuated sphere which is surrounded by a liquid but does not address itself to the dynamics of the driving environment. U.S. Pat. Nos. 4,266,442, 4,285,251 and 4,860,611, on the other hand, disclose different ways of constructing high speed rotors. However, the above referenced patents do not recognize, let alone describe, design features needed for compatibility with the environment of a motor vehicle.
High speed rotors (greater than 2500 radians per second, for example) such as used in high energy density flywheels, often require a magnetic bearing system in order to avoid the problems associated with the lubrication and cooling of mechanical bearings. Magnetic bearings require a set of sensors to measure, with adequate speed and precision, the displacement of the rotor axis from its neutral position in order for the corrective magnetic forces to be applied in a timely fashion.
U.S. Pat. Nos. 3,490,816 and 3,860,300, both of which were issued to Joseph Lyman, describe the general principles of a magnetic bearing system wherein the static load, i.e., the resting weight of a rotor, is supported by one or more permanent magnets while dynamic force generators, i. e., force coils, provide compensation for acceleration differences between the rotor and a surrounding stator. In U.S. Pat. No. 3,490,816, for example, the axial velocity is derived from a sensing coil disposed above the load bearing permanent magnet while axial displacement over relatively long time periods is sensed using an opaque piston, a light source, optical baffles and optical sensors arranged at the end of the rotor shaft opposite the permanent magnet. The system proposed in U.S. Pat. No. 3,490,816 requires two separate amplifiers, one specifically matched to the associated sensor type.
It will be appreciated that non-contacting sensors are required for high speed energy storage flywheel systems because non-contacting sensors offer long life in a high rotational speed environment. Examples of non-contacting sensors are disclosed in U.S. Pat. Nos. 5,036,236 and 5,314,868. While magnetic sensors such as those disclosed in either U.S. Pat. No. 3,490,816 or U.S. Pat. No. 5,036,236 may be used, magnetic sensors are generally degraded by changes in the material properties of the surfaces being sensed, such as the electrical resistivity and magnetic permeability of the sensed surfaces.
On the other hand, optical sensors such as those disclosed in U.S. Pat. No. 3,490,816 may be subject to degradation since such optical sensors normally include delicate, sophisticated circuit components. These supporting components would frequently be located outside of the flywheel enclosure, which would require the routing of signal lines between the optical sensors themselves and the supporting components and between the supporting components and the force generators. It will be appreciated that this would severely complicate the connections between the energy storage flywheel and the rest of the power train components. An alternative approach would be to locate the supporting components within the confines of the flywheel, which would simplify the cable routing concerns but would require additional efforts to adapt the supporting components to the environment of the interior of the flywheel, i.e., a vacuum environment.
The magnetic bearing system described in U.S. Pat. No, 3,860,300 proposes a virtually zero powered magnetic suspension system, which may provide a solution to the problem of maximizing the rundown time in the parking mode of operation. However, the system proposed in U.S. Pat. No. 3,860,300 is not up to the task of compensating for high velocity rotor displacements while maintaining adequate bearing stiffness in the driving mode of operation.
U.S. Pat. No. 4,511,190 to Caye et al. describes a magnetic bearing system for stationary systems that uses a passive method of controlling radial instability, which system is apparently incompatible with the automobile environment. U.S. Pat. No. 5,216,308 to Meeks describes an efficient combination of permanent magnets and electromagnets in radial magnetic bearings. It will be appreciated that the use of permanent magnets in the radial bearings is incompatible with the need for a long run down time in the parking mode, since the eddy currents developed in the rotor create losses which must be replaced with energy stored in the flywheel.
The present invention was motivated by a desire to correct the perceived weaknesses and identified problems associated with conventional magnetic bearing systems used with flywheel energy storage systems.